DynamoDB vs MongoDB NoSQL [closed]

Question

I know this is old, but it still comes up when you search for the comparison. We were using Mongo, have moved almost entirely to Dynamo, which is our first choice now. Not because it has more features, it doesn't. Mongo has a better query language, you can index within a structure, there's lots of little things. The superiority of Dynamo is in what the OP stated in his comment: it's easy. You don't have to take care of any servers. When you start to set up a Mongo sharded solution, it gets complicated. You can go to one of the hosting companies, but that's not cheap either. With Dynamo, if you need more throughput, you just click a button. You can write scripts to scale automatically. When it's time to upgrade Dynamo, it's done for you. That is all a lot of precious stress and time not spent. If you don't have dedicated ops people, Dynamo is excellent.

So we are now going on Dynamo by default. Mongo maybe, if the data structure is complicated enough to warrant it, but then we'd probably go back to a SQL database. Dynamo is obtuse, you really need to think about how you're going to build it, and likely you'll use Redis in Elasticcache to make it work for complex stuff. But it sure is nice to not have to take care of it. You code. That's it.

I recently migrated my MongoDB to DynamoDB, and wrote 3 blogs to share some experience and data about performance, cost.

Migrate from MongoDB to AWS DynamoDB + SimpleDB

7 Reasons You Should Use MongoDB over DynamoDB

3 Reasons You Should Use DynamoDB over MongoDB

With 500k documents, there is no reason to scale whatsoever. A typical laptop with an SSD and 8GB of ram can easily do 10s of millions of records, so if you are trying to pick because of scaling your choice doesn't really matter. I would suggest you pick what you like the most, and perhaps where you can find the most online support with.

For quick overview comparisons, I really like this website, that has many comparison pages, eg AWS DynamoDB vs MongoDB; http://db-engines.com/en/system/Amazon+DynamoDB%3BMongoDB

Short answer: Start with SQL and add NoSQL only when/if needed. (unless you don't need anything beyond very simple queries)

My personal experience: I haven't used MongoDB for queries but as of April 2015 DynamoDB is still very crippled when it comes to anything beyond the most basic key/value queries. I love it for the basic stuff but if you want query language then look to a real SQL database solution.

In DynamoDB you can query on a hash or on a hash and range key, and you can have multiple secondary global indexes. I'm doing queries on a single table with 4 possible filter parameters and sorting the results, this is supported (barely) through the use of the global secondary indexes with filter expressions. The problem comes in when you try to get the total results matching the filter, you can't just search for the first 10 items matching the filter, but rather it checks 10 items and you may get 0 valid results forcing you to keep re-scanning from the continue key - pain in the neck and consumes too much of your table read quota for a simple scenario.

To be specific about the limit problem with filters in the query, this is from the docs (http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/QueryAndScan.html#ScanQueryLimit):

In a response, DynamoDB returns all the matching results within
the scope of the Limit value. For example, if you issue a Query 
or a Scan request with a Limit value of 6 and without a filter
expression, the operation returns the first six items in the 
table that match the request parameters. If you also supply a
FilterExpression, the operation returns the items within the 
first six items in the table that match the filter requirements.

My conclusion is that queries involving FilterExpressions are only usable on very rare occasions and are not scalable because each query can easily read most or all of your of your table which consumes far too many DynamoDB read units. Once you use too many read units you'll get throttled and see poor performance.

Expert opinion: In the AWS summit on Apr 9, 2015 Brett Hollman, Manager, Solutions Architecture, AWS in his talk on scalling to your first 10 million users advocates starting with a SQL database and then using NoSQL only when and if it makes sense. Because sooner or later you'll probably need a SQL server somewhere in your stack. His slides are here: http://www.slideshare.net/AmazonWebServices/deep-dive-scaling-up-to-your-first-10-million-users See slide 28.